OT? Binoculars as field microscope

Hi all.

I have always been interested in most stuff one can find in the wild.
The latest interst has been leichens. It turns out that the leichens
look like "forests" in sub-mm height scale, and that very strong
magnification is needed to see anything of interest. So I called up
my optics shop and asked what they had for large-magnification
field use, basically field microscopes.

After some discussion, the guy asked "Have you tried using your
binoculars backwards?"

No, I had not, but I tested with my 8x20s. It turns out the binos
work beautifully as microscope. I point the eyepiece towards the
object and look through the objective. I wouldn't be surprised if
there is a magnification in the range 100-200x, depending on
the distance between the object and the eyepiece.

As I understand, this is a well-known trick. But it got me curious.
How does it work, what is this all about? Could anybody explain it
to me? Are there ways to optimize the "microscopic aspect" of a
pair of binos?

Rune

Rune Allnor wrote:
Hi all.

I have always been interested in most stuff one can find in the wild.
The latest interst has been leichens. It turns out that the leichens
look like "forests" in sub-mm height scale, and that very strong
magnification is needed to see anything of interest. So I called up
my optics shop and asked what they had for large-magnification
field use, basically field microscopes.

After some discussion, the guy asked "Have you tried using your
binoculars backwards?"

No, I had not, but I tested with my 8x20s. It turns out the binos
work beautifully as microscope. I point the eyepiece towards the
object and look through the objective. I wouldn't be surprised if
there is a magnification in the range 100-200x, depending on
the distance between the object and the eyepiece.

As I understand, this is a well-known trick. But it got me curious.
How does it work, what is this all about? Could anybody explain it
to me? Are there ways to optimize the "microscopic aspect" of a
pair of binos?

Rune

My binoculars won't magnify much, and the distortion when they do is
prohibitive. I could (but I wouldn't) get better magnification by
dismounting an eyepiece and using it directly. (I carry a 10X Hastings
triplet on my key ring.)

A compound microscope is simply* two lenses separated by more than the
sum of their focal lengths. A telescope focused at infinity consists of
two lenses separated by exactly the sum of their focal lengths, with the
shorter focal length near the eye. When focused close, the separation
increases, and some microscope action is possible. Some, but not usually
much. (I have a negative lens which can magnify a little too. People who
know a little optics know that a thin negative lens can't magnify, and I
win bets with this thick one.)

I'm surprised that your binoculars magnify as much as they do, and that
such magnification can be useful. Longitudinal magnification is the
square of lateral magnification, so a 100X enlargement implies axial
sensitivity to movement of one part 1n 10,000 to maintain focus. That is
why microscope frames need to be so rigid and why fine-focus knobs are
calibrated in microns. http://tinyurl.com/qf5th might interest you.
Decent used bench microscopes are pretty cheap. Some have built-in
illuminators; the ones I can get here need 120 volts. (There's a
transformer, so changing the bulb isn't an option.) Lichens and mosses
reveal a lot with a stereo microscope like http://tinyurl.com/n9z28

Jerry
__________________________________
* The lenses themselves are not necessarily simple.
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Rune Allnor wrote:
As I understand, this is a well-known trick. But it got me curious.
How does it work, what is this all about? Could anybody explain it
to me? Are there ways to optimize the "microscopic aspect" of a
pair of binos?

It's a bit hard to explain in text. But I'll give it a shot. In short,
whether you're using the binoculars as such, or as a microscope, you
want the real image formed by the "front" lens--the one closer to the
object--to be as close to the "rear" lens--the one closer to your eye--
so that that rear lens magnifies it as big as possible for your eye, in
exactly the same way that a magnifying lens does for a real object.

Binoculars are essentially the same as refractors, except that there are
two of them, of course, and binoculars fold the light path. So I'll
explain what's happening for refractors, and you can apply the same kind
of understanding to binoculars, yes?

We see an object, whether it's a distant galaxy or a close-up insect,
because it emits light rays. In the case of the galaxy, it emits its
"own" light rays; in the case of the insect, it's only reflecting light.
But that doesn't matter for our purposes--in both cases, light rays are
emanating from the object in all directions.

In the case of the galaxy, some of those light rays are intercepted by
the objective lens, and those rays are focused to converge in a real
image at the focal plane of the refractor. (The focal plane is an
imaginary surface; its distance from the objective is equal to the
objective's focal length, and its distance from the eyepiece--in the
opposite direction--is equal to the eyepiece's focal length.) The real
image is like a little miniature of the galaxy, floating in mid-air at
the focal plane, and it can be magnified by the eyepiece just as though
it were a real physical object there. Since the eyepiece's focal length
is so short, it's like using a powerful magnifying lens, and the galaxy
looks bigger (if not exactly bright).

Now, let's turn the refractor upside-down and point the eyepiece at the
insect. Now, light rays are emanating from the insect and being
intercepted by the eyepiece. Now, suppose the eyepiece has a focal
length of 10 mm. That means that if you have an object at infinity (or
"almost" infinity, from an optical perspective, where all astronomical
objects are), the eyepiece would form a real image, inverted, at a
distance of 10 mm from the eyepiece. That's because the light rays
being intercepted by the eyepiece are essentially parallel when they
get there.

If the object is closer than infinity--say, a meter (1000 mm) away--the
light rays aren't quite parallel when they're intercepted by the
eyepiece. They're slightly diverging, because the object is close by.
As a result, the eyepiece lenses can't quite converge them within only
10 mm; it takes a little longer, but only a little: about 10.1 mm. The
closer the object gets, though, the longer it takes for the rays to
converge to an image. At a distance of 60 mm, it takes 12 mm for them
to converge; and at a distance of 20 mm, it takes 20 mm for them to
converge. Finally, when the object is only 10 mm away--the focal length
of the eyepiece--the rays never converge at all. (Note that this is the
exact opposite of the rays coming from infinity and converging in
10 mm.) The formula--just an approximation, but close enough for us--
is

1/d + 1/r = 1/f

where d is the distance to the object, r is the distance at which the
real image is formed, and f is the focal length of the eyepiece. All
must be in the same units, of course.

If you test out this formula with f = 10 mm and d = 10.2 mm, say, you
find that r = 500 mm, approximately. If that's just a little less than
the distance between the eyepiece and the objective, then the real
image is formed just inside the objective, and you can look through it
at that real image, just as though it were a real insect (but much, much
bigger--about 50 times bigger, the ratio between 500 and 10.2).

To be sure, the objective will be closer to the real image than its
focal length, so the light rays won't exit the objective in parallel,
but gently diverging, but that's not a problem--our eyes are used to
diverging light rays. Just pull your eyes back, probably not too far,
until it's comfortable to view the magnified insect, and it'll still be
much bigger than it looks like to the unaided eye.

Hope that helps.

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

Jerry Avins skrev:

I'm surprised that your binoculars magnify as much as they do, and that
such magnification can be useful.

Well, I have no way of checking the exact numbers. Suffice it to say
that depending on the light, I can see that a strand of human hair is
hollow. I am able to see the differences in opacity across the width of

the hair. I don't know if the mag number can be derived from that. And
yes, the optical quality of those binos is decent.

I tried the same trick with my telescope. It worked, but somehow
there was some sort of optical distorsion kicking in, destroying
the image pretty early.

Rune

Rune Allnor wrote:
Hi all.

I No, I had not, but I tested with my 8x20s. It turns out the binos
work beautifully as microscope. I point the eyepiece towards the
object and look through the objective. I wouldn't be surprised if
there is a magnification in the range 100-200x, depending on
the distance between the object and the eyepiece.

I may be wrong, but I don't think it can magnify the image more than
the stated magnification of the binocular.

As I understand, this is a well-known trick. But it got me curious.
How does it work...

It works the same way a telescope works, except the object is very
close to the objective lens. The objective lens forms an inverted,
magnified image, and the other lenses magnify that image.

American Science and Surplus used to sell a contraption that converts a
small set of roof prism binoculars into a steady binocular microscope,
complete with stage for specimens, rack and pinion focus, etc. Try
them at sciplus.com.


J. Del Col

Brian Tung skrev:
Rune Allnor wrote:
As I understand, this is a well-known trick. But it got me curious.
How does it work, what is this all about? Could anybody explain it
to me? Are there ways to optimize the "microscopic aspect" of a
pair of binos?

It's a bit hard to explain in text. But I'll give it a shot. In short,

[snip]

Thanks. I'll have to spend a little bit of time contemplating your
post.
Somebody else mentioned that the magnification should not exceed
the "proper" mag factor of the binos. Is that right? Did I
over-estimate
the magnification so badly, or do the reversed binos really
magnify more?

Rune

As a very good alternative, many newer binoculars can focus fairly
closely. I recently got a pair of Pentax Papilio glasses. They come in
6.5X or 8X magnification. I got the 6.5 for its wider field of view. These
mechanically converge the objectives so they can focus closely, down to 20
inches or so. I can hold something in my hand at arms length and see it
magnified 6.5X. --Or I can look at lichens or ferns at short distances and
get very good images. It's not bad at longer ranges either.

Joe P.


"jadel" wrote
I just checked the American Science and Surplus site. They have the
device I mentioned for $39.95. Check their site index for microscopes
and accessories.
The device includes a pair of 2X lenses that double the magnification
of whatever bino you use with it. It is meant for compact roof prism
binos only.
J. Del Col

Joe Pylka wrote:
As a very good alternative, many newer binoculars can focus fairly
closely. I recently got a pair of Pentax Papilio glasses. They come in
6.5X or 8X magnification. I got the 6.5 for its wider field of view.....

I looked at those, but they were a bit pricey for the moment, so I got
a Carson close-focus monocular, 7X32, focuses down to 18 inches. Nice
little item.

Brunton sells a similar monocular, but it costs a lot more.

J. Del Col

Figure on about 3-4x for 10x binos.
There used to be field microscopes available
of decent quality, but ones I've seen recently
are toyish. Consider a company like "Peak"
out of Japan, they make some decent stuff
that can be used in the field.

jadel wrote:
Joe Pylka wrote:

As a very good alternative, many newer binoculars can focus fairly
closely. I recently got a pair of Pentax Papilio glasses. They come in
6.5X or 8X magnification. I got the 6.5 for its wider field of view.....


I looked at those, but they were a bit pricey for the moment, so I got
a Carson close-focus monocular, 7X32, focuses down to 18 inches. Nice
little item.

Brunton sells a similar monocular, but it costs a lot more.

I think that the best way to use a monocular or one side of a binocular
as a magnifier is with an achromat as a close-up attachment in front of
the regular objective and the instrument in its usual orientation. A 75
mm achromat in front of a an 8x25 instrument yields about 25 power. I
can send one to Rune if he wants it, but without an attachment means
unless we communicate first.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯